Enthalpies of hydrolysis of glutamine and asparagine and of ionization of glutamic and aspartic acids

Abstract

Direct measurements are described of the heats evolved upon the enzymic hydrolysis of the amide groups of glutamine and asparagine in aqueous solution. At pH 7, the AH values are -5160 [plus or minus] 70 and -5710 [plus or minus] 100 cal/ mole respectively. The heats of ionization of the secondary carboxyl groups of glutamic and aspartic acids have also been measured; the respective 4H values are 950 [plus or minus] 30 and 1530 [plus or minus] 35 cal/mole. The values found for asparagine are consistent with the independent values for the heats of combustion and solutions of asparagine and aspartic acid. In view of the unexpected difference between the heats of hydrolysis of the 2 amides and also between the 2 heats of ionization, a satisfactory value for the free energy of hydrolysis of glutamine cannot be obtained by analogy with the data for asparagine. More data are therefore needed to obtain an accurate value for the free energy change of hydrolysis of adenosine triphosphate by a calculation based on the equilibrium of the glutamine synthetase reaction.